Metal porphyrins catalyse luminol chemiluminescence at pH 13 without added peroxide. The effects of 22 different surface active compounds on this reaction were studied using six metal porphyrins and one metal porphyrin conjugate. The most active catalyst was PAn-meso-tetra(4-sulphonatophenyl)porphine. Tween-20 enhanced the activity o f this catalyst best at a Tween-20 t o luminol ratio of 74: 1. However, lauryl sulphate enhanced best at an optimum lauryl sulphate t o luminol ratio o f over 1OOO:l and both detergents enhanced the reaction when present below their critical micelle concentrations. Negatively charged aliphatic compounds such as fatty acids enhanced the reaction but positive-charged aliphatic compounds inhibited it. Small differences in enhancer structure resulted in differing enhancement. For example, linoleic acid enhanced Mn-meso-tetraphenyl porphine more than 10-fold, yet linolenic acid inhibited this catalyst. Conjugation of a metal porphyrin t o antibody did not influence its enhancement by detergents. The results indicate that the enhancement mechanism does not require formation o f pure detergent micelles but that direct association between enhancer and catalyst may be important.